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Steam Engine

06/14/2011 7:47 PM

Can anyone explain, Rankine cycle steam engine is known to be very inefficient. This inefficiency is due to hugh amount of latent heat has to be dissipated after condensation. Since it is so, why can't steam engine operates without condensation, i.e, the expanded steam (still in vapour state, but in much lower pressure) is pumped back to the firebox to absorb heat again to high pressure again?

Thank you.

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#1

Re: Steam Engine

06/14/2011 8:00 PM

There's some reason, having to do with phase change, I think. This is all fuzzy. Maybe it's something else. Why don't you investigate, and let us know why and report back on that.

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#2

Re: Steam Engine

06/14/2011 9:55 PM

Condensation makes the steam cycle more efficient (rather than less), by lowering the "cold sink" temperature. (Railroad steam engines were generally noncondensing, whereas ships' engines were/are often of condensing type.)

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#3
In reply to #2

Re: Steam Engine

06/14/2011 11:14 PM

Hi Tornado, thanks for your reply.

Railroad steam engine is non condensing, that's is why it is even more inefficient. The boiler has to raise water temp from ambient to boiling pt then to vapour.

If you said you need condensation to achieve temp gradient, say from 500 deg C (steam) to 99 deg C (condensate to liquid phase), compare this to the situation where there is no phase change (condensation) , say from 500 deg C to 101 deg C, i.e. both having about the same temp gradient, the latter case is without latent heat loss. So, the latter case it much more efficient, therefore it is more desirable. But in reality, this is not the case, all close cycle steam engine have condensation.

This is something I can't quite figure out .

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#6
In reply to #3

Re: Steam Engine

06/15/2011 7:37 AM

I think it might have something to do with the difficulty in pumping low-pressure steam into the high-pressure boiler. You could be using more energy than you are saving.

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#27
In reply to #6

Re: Steam Engine

06/18/2011 1:26 PM

Locomotive steam engines used vacuum eductors to move makeup water from the water tankcar to the boiler, another heat loss, because higher pressure steam had to be utilized. The piston engines still emitted steam during the exhaust stroke. Most power generation plants utilize condensers to result in a condensate nearer 37-40 C degrees. Unfortunately for the Rankine cycle, all the heat under the condensation line on the entropy diagram is waste heat.

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#4

Re: Steam Engine

06/15/2011 2:07 AM

Pumping low pressure vapor into boiler pressure is not especially easy, and even when it gets there, not much energy will be added to it.

The expended vapor from a live steam process may be well over 100°C, but a vacuum condenser may result in vapor/condensate substantially below 100°C.

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#5

Re: Steam Engine

06/15/2011 3:20 AM

Some locomotives used exhaust steam to heat the boiler feed water to increase efficiency. But this in it's self caused problems in that a feed water pump had to be provided as normal injectors couldn't be used to feed the boiler. It was tried by several British railway companies with limited success.

http://en.wikipedia.org/wiki/Franco-Crosti_boiler

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#7

Re: Steam Engine

06/15/2011 8:03 AM

Hey Bravo. This Steam Turbine Training may lead to a better understanding. Click link for video. On the YouTube channel there is also a Mechanical training video playlist with more videos too.

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#8

Re: Steam Engine

06/15/2011 3:20 PM

Holzfeller and Tornado have it right. It isn't required to condense the exhaust steam, but for the same exhaust (sink) conditions it is actually MORE efficient to condense the exhaust.

Take a look at a temperature vs entropy diagram for a Carnot or Rankine cycle. The area under the "upper" curve (across the boiler) is the input. The area under the "lower" curve (across the condenser) is the output. The difference between them is the work out. So if we pumped steam back to the boiler, we'd see the lower curve change so that the "work out" area has decreased. Less work for the same input is less efficient!

It seems counter-intuitive, but that's how it works!

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#9

It's the pumping efficiency

06/15/2011 5:20 PM

First of all, "very inefficient" is a relative term. The Rankine cycle is actually quite efficient as practical thermodynamic cycles go.

A quick Wikipedia scan of the Rankine Cycle page yields the answer to your question:

http://en.wikipedia.org/wiki/Rankine_cycle

"A pump is used to pressurize the working fluid received from the condenser as a liquid instead of as a gas. All of the energy in pumping the working fluid through the complete cycle is lost, as is all of the energy of vaporization of the working fluid, in the boiler. This energy is lost to the cycle in that first, no condensation takes place in the turbine; all of the vaporization energy is rejected from the cycle through the condenser. But pumping the working fluid through the cycle as a liquid requires a very small fraction of the energy needed to transport it as compared to compressing the working fluid as a gas in a compressor (as in the Carnot cycle)."

.... "One of the principal advantages the Rankine cycle holds over others is that during the compression stage relatively little work is required to drive the pump, the working fluid being in its liquid phase at this point. By condensing the fluid, the work required by the pump consumes only 1% to 3% of the turbine power and contributes to a much higher efficiency for a real cycle."

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#10
In reply to #9

Re: It's the pumping efficiency

06/15/2011 7:18 PM

Hi Steve and Chaoticintellect,

Much thanks for your explanation. Now I got it.

Because it is pumping back liquid , the volume is much smaller, so less work is needed to pump back to the pressurised boiler. The work to do pumping is PV.

How come I cant figure that out !?

Thanks again guys!!

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#13
In reply to #10

Re: It's the pumping efficiency

06/16/2011 7:57 AM

Usually, (Steam Turbines), they recycle the steam from different stages, back to the boiler but not pumping it into the boiler! They pass it into pipes that are heated by the boiler exhaust gases: Super heating. This is more efficient since the latent heat is not wasted by condensing the steam. The re-heating process elevates the temperature/pressure and allow additional work at less cost. This is done once or twice in large turbines (in the 300 to 600 MW).

You mentioned 500°C and that looks large (?) for a steam engine.

Therefore, maybe re-superheating the steam without re-compressing it into the boiler can be done...!?

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#16
In reply to #13

Re: It's the pumping efficiency

06/16/2011 12:04 PM

In marine applications saturated steam is taken from the steam drum to the superheater tubes and some boilers have a separate superheater furnace. This superheated steam is used to drive the turbines and the steam goes from boiler pressure (e.g. 600-1200psi) down to 30'' of vacuum in the condenser, via the HP and LP turbines. The condensate is then pumped via turbo-driven extraction pumps back to feed water tanks. This is called a closed feed system. Water makers are required on board ships to keep up the supply of feed water

Most auxiliary pumps are turbo-driven, as are the ship's gen sets, these are usually self condensing, again condensate being returned to feed water tanks.

If saturated steam is required this can either be taken from a stop valve on the water drum of the boiler or via a de-superheater.

To improve overall efficiency economisers are fitted in the boiler uptakes to pre-heat the feedwater before it enters the boiler via the feed water regulator.

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#18
In reply to #9

Re: It's the pumping efficiency

06/16/2011 3:23 PM

I recently heard of a project in Belgium for a 7000 psi Rankine cycle, coal-fired, vacuum condenser (as is the normal practice). The relative efficiency (heat rate per kW-h will be low I suspect) should be high.

To really blow your mind, Sandia Laboratories recently published a news release about their supercritical carbon dioxide Brayton cycle (in this case a closed Brayton cycle). Where the density of the low temperature, lower pressure side is nearly sufficient to use a liquid type pump as a compressor. The efficiency even for a small pilot plant was reported to be at 140% of present conventional air-fuel Brayton open cycle jet engines.

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#19
In reply to #18

Re: It's the pumping efficiency

06/16/2011 4:51 PM

Re: The efficiency even for a small pilot plant was reported to be at 140% of present conventional air-fuel Brayton open cycle jet engines.

So, we finally see a real "over unity" energy scheme. ;-) (I'm just kidding) On a serious note, if anybody knows the approximate or typical efficiency of "present conventional air-fuel Brayton open cycle jet engines", it would be nice to know that. Maybe even multiply it by 1.4 to see what the future might hold.

Oh, heck, let me google: ok, the wikipedia article on the Brayton cycle doesn't help me too much--a chart on that page indicates efficiency varying between about 40% and 60% as the pressure ratio varies between about 2.5 and 22.5. (Like I say, that doesn't help me much...)

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#20
In reply to #19

Re: It's the pumping efficiency

06/16/2011 5:25 PM

Basically, I think the best simple cycle jet engine can do about 40% efficiency, with most of the energy produced in the expansion going to drive the compressor (about 60%). 140% of that would be approximately 56%. 3412 BTU is equivalent to precisely 1 kW-hr. The heat rate of a 40% efficient cycle would be 8030 BTU/kW-hr.

The heat rate of a 56% efficient cycle would be 5736 BTU/kW-hr, a remarkable difference if the fuel cost is paid from your checking account. Granted, one usually thinks of economy of scale when it comes to electric power costs, but this is one new technology that can and will eventually be scaled up, and can derive heat from a goodly number of sources, not just fossil.

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#21
In reply to #20

Re: It's the pumping efficiency

06/16/2011 6:47 PM

Thanks!

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#11

Re: Steam Engine

06/15/2011 11:46 PM

See Animation_-_Power_Plant_Fundamentals including Rankine cycle and more.

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#12
In reply to #11

Re: Steam Engine

06/16/2011 12:36 AM

Guruji Halim Galala. Thank you. i needed it very badly for my various seminars. i rated this. mail be other animation sites like this. i will be obliged.

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#14
In reply to #12

Re: Steam Engine

06/16/2011 10:23 AM

ducon, you are welcome.

That is for you: Animation_-_Air_Cooler_Bank_by GEA.

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#15

Re: Steam Engine

06/16/2011 10:55 AM

Just think about the question as you asked it and sketch out what you propose. You obviously can't go back to the same boiler that the steam has come from since it would be at a much higher pressure and the exhaust steam couldn't enter the boiler drum(much like making water flow up hill). So you propose to put the exhaust steam into some sort of fired heater to heat it up and therefore raise its pressure in order to re-use it at that higher pressure and return it to the high pressure side of the steam engine.

What prevents that higher pressure that the returning steam is raised to from flowing or pushing backwards to the engine exhaust side and equalizing the pressure in that whole line?

It is the expansion process that goes on in the engine that provides power by removing the enthalpy or entropy (I forget which is which and I'm too lazy to look it up). A triple expansion engine has a high, medium and low stage. The low stage feeds into a condenser or a boiler feed water heat exchanger to recover some of the latent heat thereby improving the overall efficiency. That exhaust from the third stage is a very wet steam at that point, often semi-condensed and serves to lubricate the piston rings.

I think a simple flow diagram or schematic would show you the implausibility of the arrangement that you are suggesting.

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#17

Re: Steam Engine

06/16/2011 12:12 PM

In the boiler, the feedwater changes to steam at a much higher pressure. (The process of changing the water to steam increases the pressure.) By cooling the steam back to condensate in the condenser, the volume of the steam as it changes back to water (condensate) drops considerably thereby creating a vacuum which helps to pull the steam from the boiler through the turbine. This reduction process of going to condensate from steam actually is what makes this inefficient process so efficient. If your cooling process in the condenser is not very successful, you have a problem with "back pressure" which makes for an inefficient Rankine cycle. Cooling the steam to a much lower temperature reduces this back pressure. (In other words, it is a good thing.)

In most large Rankine Cycle boilers, feedwater heaters are used to reclaim the latent heat by taking extraction steam from the different stages of the turbine and preheat the condensate / feedwater before going back into the boiler. The OP refers to the huge amount of latent heat that has to be dissipated after condensation, but it actually needs to be dissipated before condensation by passing through the shell side of the feedwater heaters as extraction steam. The extraction steam passing through the feedwater heaters increases the efficiency of the cycle by getting the temperature of the feedwater closer to the temperature of the boiler thereby reducing the amount of thermal input required to turn it back to steam.

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#22

Re: Steam Engine

06/17/2011 5:31 AM

Another follow-up question: Since the inefficiency of steam engine is due to latent heat loss, which is very high for water, then why can't we use a fluid that has low latent heat such as Refigerant (of course for close cycle) , which has 10 times lower latent heat of vapourisation ?

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#23
In reply to #22

Re: Steam Engine

06/17/2011 9:49 AM

It isn't quite correct to say that latent heat loss is a cause of inefficiency. As pointed out earlier, condensing the vapour makes the cycle more efficient. If there is some way to use that rejected heat, that will further improve the overall process efficiency.

That aside, you still have a valid question regarding why we use water and not another fluid. It comes down to money. Steam plants are not hermetically sealed and leak water and steam from various locations. There would be an enormous cost in completely sealing the system similar to your AC system. Water is widely available, non-hazardous, inert, and relatively cheap.

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#24
In reply to #23

Re: Steam Engine

06/17/2011 10:49 AM

GA

A possibility perhaps (just suggestion): If a refrigerant type circuit is used to pump the heat from the steam condenser circuit (Latent heat) and transfer it back to the condensed steam before sending it back into the boiler under pressure (The pressurised water will allow it to collect heat to a higher degree without evaporrating), even though you are consuming some power, but the heat transfer is high efficiency (> 200%). This needs more scrutiny but I thing there might be a potential to increase the overall efficiency. (??)

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#25
In reply to #24

Re: Steam Engine

06/17/2011 3:26 PM

There is a concept in Rankine cycles called regeneration that I studied. I have worked with several designs but have no real world experience with these. The basic idea is this... extract a portion of steam before it has fully expanded through the turbine (this is common and is used for heating or auxiliary processes) and condense it with the incoming feedwater (thereby preheating it).

The loss of power from not expanded it more than made up for by recycling the latent heat. For the design I studied, I seem to remember an increase in power output by about 5% for the same heat input.

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#26

Re: Steam Engine

06/17/2011 10:32 PM

It is clear to me now, condensation make it more efficient as compared to non -condensation in (open circuit system), but in itself is a major but unavoidable cause of inefficiency. For close circuit, since you need to re-cycle the expanded steam back to the boiler , condensation method requires less energy than pumping back uncondensed steam to boiler.

If someone can find another path to re-cycle the expanded steam back to the boiler without condensation, that will be a hugh break-through, all of we will enjoy lower electricity bill.

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